I‘d say „Fill“ could be called „Pre-Opacity“, because it applies transparency before any layer effect as opposed to „Opacity“, which applies transparency after all layer effects. You could call that „Post-Opacity“.

Although you can achieve a similar effect in some cases, fill layers in Photo are something else entirely than layer fill in Photoshop.
Another question, is there a way to make level adjustments on masks? I often use it to sharpen masks that are too blurry.

Thank you Walt. This does seem to have a similar effect, but I haven't really grasped the functionality of fill layers yet.
I guess I will have to read the manual, which I usually never do. Photo is really a great app, especially on the iPad, but after using Photoshop for about 20 years there are quite a few differences with handling things that makes working with Photo sometimes frustrating. Especially masks.

Hello.
I've been using Photo for quite a while now on iPad and have just purchased the Mac version.
So far a great app, but there is one major (at least for me) feature missing – the fill slider for layers, like in Photoshop.
I use fill far more often than opacity in Photoshop, because the way blend modes and effects interact with the background better suits my editing style and requirements.
Here's an example of the different ways blending is handled when using fill vs. opacity:
I don't seem to be able to replicate that effect or behavior in Affinity Photo by any means.
Please implement this functionality in Photo for both Desktop and iPad.
Thank you!
Michael